1. Field of the Invention
The present invention relates in general to brake systems of wheeled motor vehicles, and more particularly to vehicle brake control systems of a type that increases the brake force applied to road wheels by actively feeding a brake fluid to a brake fluid circuit with the aid of an electric fluid pump.
2. Description of the Related Art
One of the vehicle brake control systems of the above-mentioned type is disclosed in Laid-open Japanese Patent Application (Tokkaihei) 9-226555. In this known system, a computer aided pressure control valve is employed, which suitably controls the hydraulic pressure applied to the wheel cylinder of each road wheel in accordance with running condition of an associated motor vehicle and driver's brake operation. With this control, undesired lock of the road wheels can be avoided (viz., anti-locking brake control (viz., ABS)) and unstable movement of the vehicle can be suppressed or at least minimized.
In this brake control, for reducing the hydraulic pressure in the wheel cylinder, the brake fluid is driven from the wheel cylinder to a reservoir. Furthermore, under pressure control, the brake fluid is fed back to the wheel cylinder as the occasion arises by operating an electric fluid pump.
Furthermore, upon completion of the brake control, the brake fluid left in the reservoir is enforcedly driven back to a master cylinder by operating the electric fluid pump for a certain time. This flow back operation effected by the electric fluid pump aims for keeping the capacity of the reservoir sufficient for a subsequent pressure control. That is, if a certain amount of brake fluid is left in the reservoir and thus the practical capacity of the same is not sufficient, the wheel cylinder can not have a sufficiently large pressure control range. Thus, to carry out the subsequent pressure control assuredly, the brake fluid left in the reservoir is enforcedly driven back to the master cylinder by operating the electric fluid pump as is mentioned hereinabove.